This invention relates generally to the field of ophthalmic lenses and, more particularly, to toric intraocular lenses (e.g. pseudophakic IOL, AC phakic IOL, PC phakic IOL, Iris-fixed IOL and etc.).
The human eye in its simplest terms functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of a crystalline lens onto a retina. The quality of the focused image depends on many factors including the size and shape of the eye, and the transparency of the cornea and the lens.
The optical power of the eye is determined by the optical power of the cornea and the crystalline lens. In the normal, healthy eye, sharp images are formed on the retina (emmetropia). In many eyes, images are either formed in front of the retina because the eye is abnormally long (axial myopia), or formed in back of the retina because the eye is abnormally short (axial hyperopia). The cornea and crystalline lens also may be non-spherical and regular but asymmetric or toric, resulting in an uncompensated cylindrical refractive error referred to as corneal and lenticular astigmatisms, which in combination give ocular astigmatism. Finally, if a surgical procedure is performed on the eye, the procedure itself can induce corneal astigmatism.
These refractive errors can all be corrected by the use of a refractive implant, when the natural lens is left in place, or by an intraocular lens (IOL) that is used to replace the natural lens. With respect to a toric implant, the magnitude of the cylindrical power of the implant and the exact alignment of the implant with the naturally occurring or induced asymmetry of the cornea/eye is necessary in order to correct the regular astigmatism of the cornea/eye. The importance of the alignment of the axis of the implant with that of the cornea/eye is important because even with matching magnitudes of cylinder, any misalignment of the axis results in unintended residual refractive errors with both sphere and cylinder. Thus, misalignment of the axis of cylinder of the implant with that of the cornea/eye is detrimental to the overall goal of optimum retinal image formation. The criticality of the needed alignment depends upon the magnitude of corneal/ocular cylinder, especially for large magnitude of corneal/ocular cylinder.
One prior art method for predicting the required spherical IOL power based on surgical techniques is disclosed in U.S. Pat. No. 5,709,218 (Holladay, et al.). This method, however, does not use power vector analysis to determine the correct power or orientation of a toric implant. There are various mathematical models available to assess surgically induced astigmatism. Authors such as Thibos, Holladay, Horner, Cravy and Koch have published papers on the use of power vector analysis to evaluate induced refractive error by Lasik surgery. The above methods provided the assessments for correction effectiveness. They, however, failed to provide a direct guidance for surgeons in terms of how to do surgery correctly to optimal refractive outcomes. In addition, these models do not account for the location of the lens within the eye.
Accordingly, a need continues to exist for a method of calculating the predicted post-operative refractive error in an eye that takes into account both naturally occurring and surgically induced astigmatic errors.